Diagnostic and therapeutic ultrasound catheters have been designed for use inside many areas of the human body. In the cardiovascular system, two common diagnostic ultrasound methods are intravascular ultrasound (IVUS) and intracardiac echocardiography (ICE). Typically a single rotating transducer or an array of transducer elements is used to transmit ultrasound at the tips of the catheters. The same transducers (or separate transducers) are used to receive echoes from the tissue. A signal generated from the echoes is transferred to a console which allows for the processing, storing, display, or manipulation of the ultrasound-related data.
IVUS catheters are typically used in the large and small blood vessels (arteries or veins) of the body, and are almost always delivered over a guidewire having a flexible tip. ICE catheters are usually used to image the chambers of the heart and surrounding structures. Commercially-available ICE catheters are not designed to be delivered over a guidewire, but instead have distal ends which can be articulated by a steering mechanism located in a handle at the proximal end of the catheter.
One type of ICE catheter (EP Medsystems ViewFlex™ Intracardiac Ultrasound Deflectable catheter) has a distal articulation in a single plane (both directions), operated by a single wheel that rotates about the lengthwise axis of the handle. The wheel is turned to a specific position for the desired catheter shape, staying in place due to the inherent friction on the wheel mechanism. The catheter is torquable, and can be rotated with the handle to aid steering in a second plane. The motions required to simultaneously torque and rotate the catheter often require two-handed operation.
Another type of ICE catheter (Siemens/ACUSON AcuNav™ Ultrasound Catheter) has an additional steering plane, and each steering plane is utilized by turning one of two corresponding wheels on the handle. These wheels rotate about the lengthwise axis of the handle. A third wheel, which also rotates about the lengthwise axis of the handle, is a locking mechanism for freezing each of the two steering wheels in its respective orientation. The entire catheter need not be torqued. However, unlocking the locking mechanism unlocks the steering wheels for both of the steering planes/wheels at the same time. While the physician is steering the catheter in one plane, the orientation of the other plane may spontaneously change to an undesired orientation. Though the two steering planes allow a large combination of possible catheter configurations, the simultaneous steering of both planes in order to achieve a specific configuration can be difficult to visualize and coordinate. Further the signal lines are formed on a flex cable with a rectangular cross-section and assembled without radial symmetry. The flex cable runs along the length of the AcuNav™ catheter behind the ultrasound transducers, causing the catheter to have different bending stiffness along different bending planes of the catheter, making the catheter more difficult to manipulate or maneuver.
Both the ViewFlex and the AcuNav catheters utilize a linear array of multiple transducer elements, e.g., 64 elements, at their tips. The design of the catheter requires 64 channels in the ultrasound console in order to process each element in parallel. This requires that the catheter shaft contain at least that many conductors to carry the signals to the proximal end. There is a limit (due to conductor size, dielectric properties, etc.) to how small the catheter shaft can be made using this design. For example, the AcuNav is currently an 8 F catheter and the ViewFlex is a 9 F catheter. In order to place these catheters within the vascular system of the patient, punctures of a diameter larger than the catheter shaft size are required. These are relatively large punctures, especially when inserted arterially, and may take longer to close or heal as well as have a greater number of related complications. In addition, because many of the cases performed in the cardiac catheter laboratory or electrophysiology laboratory require multiple catheters, having a large diameter ICE catheter can cause a “traffic jam” inside the anatomy.